With the exception of some products with special applications and materials, most of the industrial products that use metallic materials are available in a coated state. The purpose of coating is not only to enhance the aesthetic appearance but also to prevent oxidation, that is, corrosion, which metals are destined to go through. Here, the coating materials that are used for metallic materials can be classified into various types depending on the coating method or the components, and a coating material is selected in accordance with the performance required of the material to be coated, or the usable coating method. Here, when the material to be coated has a complicated structure and requires a high level of corrosion resistance, such as in the case of an automotive body, it is important to secure a coating film thickness at the interior of a pocket structure, which is a characteristic called throwing power.
A method commonly used to secure corrosion resistance in the interior of a pocket structure involves a combination of a zinc phosphate treatment, which is a chemical conversion treatment for coating foundation, and a cationic electrodeposition coating. In both of these methods, since the chemical conversion treatment and coating are performed by immersing the material to be coated in a treatment bath, the chemical conversion treatment solution and the coating material can be brought into contact even with the interior of a pocket structure. However, the zinc phosphate treatment process includes hot water rinsing → preliminary degreasing → degreasing → multistage water rinsing (normally 2 to 3 stages) → surface conditioning → coating film chemical conversion → multistage water rinsing (normally 2 to 3 stages) → ion-exchanged water rinsing, and the cationic electrodeposition coating process includes electrodeposition coating → multistage water rinsing (normally 3 to 5 stages) → ion-exchanged water rinsing → and baking. Thus, the treatment processes are very long, and for example, in the case of an automotive body, the process length may exceed 200 m.
In the zinc phosphate treatment process, as it is conventionally known, the generation of an iron phosphate sludge, which occurs as a side reaction of the coating film deposition reaction, cannot be avoided, and an improvement is desired from the viewpoint of environmental problems. Furthermore, although there have been improvements in the current coating materials for cationic electrodeposition, from the standpoint of the mechanism in which the coating film is deposited as a result of electrolysis and the coating film is thrown due to the electrical resistance of the deposited coating, there is an unavoidable problem that there occurs a difference in the thickness between the film on the outer panel where the coating film is initially deposited, and the film at the interior of the pocket structure where the coating film is deposited later.
Thus, technologies have been suggested for long, which are intended to solve the problem of the generation of iron phosphate sludge and the problem of the coating film thickness at the interior of pocket structures, while attempting to shorten the process, by depositing an organic coating film through a chemical reaction. Such compositions are called as autodeposition compositions, self-precipitating compositions, or self-deposition compositions.
For example, Patent Document 1, cited below, relates to an autodeposition composition using a vinylidene chloride copolymer. Since vinylidene chloride resins are excellent in the moisture-proofing properties, moisture resistance and gas barrier properties, when used in coating film formation, the resins have a remarkable action of suppressing corrosion. However vinylidene chloride resins have very low heat resistance, as is well known. Thus, Patent Document 1 discloses that heat resistance can be improved by copolymerizing a vinylidene chloride monomer with a comonomer, for example, an acrylic comonomer, and thereby inserting a thermally stable comonomer into the chain. However, even if a stable moiety is inserted into the chain, it is impossible to fundamentally improve the low heat resistance of the basic vinylidene chloride structure. Therefore, the autodeposition technology utilizing vinylidene chloride cannot be used for metallic materials that are used in an environment subject to the exposure to high temperatures, and also has a problem that overcoating by baking finish cannot be achieved on a coating film formed by autodeposition.
Many autodeposition compositions have also been disclosed, which do not utilize vinylidene chloride. Examples of the resin component that is used in autodeposition compositions in addition to vinylidene chloride, include, as cited in Patent Documents 2, 3 and 4, cited below, styrenebutadiene, acrylic polymers and acrylic copolymers, polyvinyl chloride, polyethylene, polytetrafluoroethylene, acrylonitrile butadiene, and urethane resins.
However, in all of these methods, the corrosion resistance of the autodeposition coating films was markedly low as compared with the coating films using vinylidene chloride. Thus, in order to enhance the corrosion resistance, there has been a need to perform, after the autodeposition coating process, a post-treatment using chromium compounds, the use of which is currently restricted due to concerns about environmental problems, as disclosed in Patent Document 3, cited below.
Thus, in recent years, an autodeposition composition combining an epoxy resin and a crosslinking agent has been proposed, as disclosed in Patent Document 5, cited below. However, when the inventors of the present invention conducted an investigation to verify the effects of the aforementioned invention, the inventors found that the autodeposition coating film utilizing an epoxy resin still cannot be said to have sufficient corrosion resistance, and the proposed autodeposition coating film has markedly low adhesiveness to solvent coating materials and has a fatal flaw that overcoating cannot be achieved thereon.
Patent Documents 6 and 7, cited below, disclose aqueous coating compositions containing a water-dispersible phenolic resin and a softening agent polymer, characterized by being capable of self-attachment on a metal support. However, because the autodeposition coating films prior to baking as obtainable by these methods contain large amounts of water, the coating films cannot be rinsed with water before baking. Accordingly, there will be no problem if the material to be coated is a flat plate; however, in the case of a material having a pocket structure, because the coating material remaining in the interior of the pocket structure cannot be washed out, serious flaws occur, which markedly affect the corrosion resistance, such as swelling and peeling of the coating film after baking.
Surface treatment technologies utilizing tannic acid have also been proposed in the past. For example, Patent Document 8, cited below, discloses a method of forming a protective coating film for metal surfaces, which includes treating a metal surface with a treatment solution obtained by adding 0.1 to 20% by weight of tannin into an aqueous solution of a water-soluble or water-dispersible organic polymer. Furthermore, Patent Document 9, cited below, discloses a chemical conversion treatment solution for pre-coating treatment containing titanium hydrofluoride, zirconium hydrofluoride, silica, tannic acid and a water-dispersible organic resin, characterized in that the mass ratio of tannic acid to the water-dispersible organic resin is 100:0.5 to 100:15, and the mass ratio, in terms of dry mass, of the inorganic substances to the total mass of tannic acid and the water-dispersible organic resin is 1:0.5 to 1:2.
Both of these methods are related to a dry-in-place type surface treatment of performing film formation by squeezing the treatment solution adhering to the surface of a metallic material to be treated, with a roll or the like to control the amount adhered, and then drying the surface. Accordingly, in the methods disclosed in the aforementioned documents, the film thickness thus obtainable is very small, and since these methods do not involve a method of depositing a coating film by way of a chemical reaction, it has been impossible to deposit a coating film at the interior of a pocket structure.
Furthermore, Patent Document 10, cited below, discloses a urethane-based aqueous adhesive composition which has a urethane polymer obtainable by polymerizing (A) a polyisocyanate compound, (B) a polyol compound having two or more hydroxyl groups, and (C) a functional compound having two or more active hydrogen atoms that are capable of reacting with isocyanate groups and having one or more hydrophilic groups selected from a carboxyl group and a sulfonyl group, dispersed in water, while the composition is characterized in that a polytannic acid compound is added to the composition in an amount of 0.5 to 20 parts by weight relative to 100 parts by weight of the urethane polymer. However, the current patent document relates to an adhesive composition, and the document discloses nothing about the treatment solution composition or method in connection with an autodeposition composition.
In addition, Patent Document 11, cited below, discloses a surface treated metal material obtained by subjecting a metal material to a treatment solution containing a water-soluble thermosetting resin, a crosslinking agent and a polyhydric phenol compound, wherein the water-soluble thermosetting resin is selected from a polyester resin, an epoxy resin and a urethane resin; the crosslinking agent is selected from an amino resin and an isocyanate; the polyhydric phenol compound is tannic acid and is contained in an amount of equal to or greater than 0.1% by mass and equal to or less than 50% by mass relative to the total solids content; and the surface treated steel plate is a Zn-based plated steel plate. However, the current patent document is also related to a Dry-in-place type surface treatment, and the metallic material that is considered as the subject of treatment is limited to a Zn-based plated steel plate.
Accordingly, in the prior art, it has been impossible to provide an autodeposition coating film which allows shortening of the process length as compared with a coating process based on the combination of zinc phosphate treatment and electrodeposition coating, does not generate any environmentally harmful by-products such as sludge, has excellent throwing power at the interior of a pocket structure, does not use environmentally harmful components such as chromium compounds, has corrosion resistance, and allows overcoating of a baking finish on the obtained coating film. It has also been impossible with the conventional technologies utilizing tannin, to obtain a large film thickness that would make it possible to substitute electrodeposition coating.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 60-58474
Patent Document 2: JP-A No. 47-32039
Patent Document 3: JP-A No. 48-13428
Patent Document 4: JP-A No. 61-168673
Patent Document 5: JP-A No. 2003-176449
Patent Document 6: Japanese Patent Application National Publication (Laid-Open) No. 2002-501100
Patent Document 7: Japanese Patent Application National Publication (Laid-Open) No. 2002-501124
Patent Document 8: JP-A No. 53-116240
Patent Document 9: JP-A No. 2002-266081
Patent Document 10: JP-A No. 8-92540
Patent Document 11: JP-A No. 2003-301274